1. Technical Field
The present invention generally relates to techniques for generating convenient supply voltages for electronic circuits of portable electronic devices, and more particularly to a DC-DC converter having a step-up converter supplying a step-down converter, also called a DC-DC step-down converter.
It finds applications, in particular, in electronic circuits which may be used in battery-powered electronic devices such as cellular phones, smart phones, digital walk-men, portable computers, PDA and similar portable electronic devices.
2. Related Art
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
New technology batteries, so called low “cut-off” batteries, used in portable electronic devices, deliver low voltages, e.g. as low as 2.5 Volts, compared to standard system higher voltage values of, for instance, ca. 3.6 Volts.
It is also desirable to provide efficient power circuitry to ensure a maximum duration between device battery charges. In particular, applications wherein the output voltage is amplitude modulated rely on quickly changing voltage regulators, for instance using DC-DC down-converting structures of the Step-Down type. Indeed, in power amplifiers, efficiency varies with the RF signal amplitude. Usually the yield is maximal at full power and drops rapidly when the amplitude of the modulated signal decreases, as a larger part of the supply voltage is not used and is thus dissipated in transistors.
Known regulated Step-Down DC-DC converters may partly compensate for this drawback and save battery power by reducing the supply voltage used for driving the RF power amplifier by tracking the signal amplitude with an efficient power supply modulator having a quick response. This technique is known as “envelope tracking”. But, eventually, the standard Step-Down converter cannot output a voltage superior to the supply voltage.
Most circuits within said devices do not require a high voltage to achieve satisfying operating conditions, with values as low as 2.5 Volts proving sufficient. However, some circuits within said portable electronic devices require a high voltage to generate high power, for instance the Radio Frequency Power Amplifier (RFPA) module of a mobile phone when transmitting data modulated and carried by an RF signal, via a radio-frequency (RF) antenna of the mobile phone. In instances such as those of GSM applications, where the RFPA requires at least a 3.3V supply to generate a specified maximum power value, the RFPA output may be reduced or the device shut down if this is impossible.
One method of achieving higher voltage in battery-powered portable electronic devices is the stacking of cells in series. However, space constraints often result in stacking insufficient to achieve the required high voltage.
Another method is based on the use of DC-DC up-converters, which can be realized, as switched-capacitor and inductive converters, to generate of a high supply voltage from the low battery voltage, namely a supply voltage of higher value than said battery voltage.
For instance, a Step-Up converter may be introduced into a circuit to raise the supply voltage from the battery. Unfortunately, Step-Up converters have very low bandwidth and higher output ripple than linear converters. Indeed, as they produce the output signal as a pulse-width modulated (PWM) signal by switching between a supplied voltage of the battery and a ground voltage value, a high level of ripples is generally obtained, even after filtering. Low ripple behaviour is however a stringent parameter, especially for DC-DC converters dedicated to supply power for RF applications, because the ripples may create interferences with RF signals. They thus decrease the signal-to-noise ratio (SNR) of the RF signal emitted by the portable electronic device. Low bandwidth may further result in a stringent limitation if the circuit designer wants to use the “envelope tracking” technique to enhance the RF power amplifier efficiency. Therefore, a compromise generally has to be found between the bandwidth of the converter, the amplitude of the output ripples, and the switching frequency of the PWM signal.
One solution to deal with ripple and bandwidth is to use a cascaded architecture where a Step-Up DC-DC converter supplies a Step-Down converter (inherently having high bandwidth and low ripple) and connect the RF power amplifier to the output of the Step-Down converter. Such a solution may provide high efficiency for a wide range of conversion ratios. In particular, this modular architecture is suitable for providing modulation with the correct bandwidth and spurious attenuation.
However, adding a Step-Up control loop to the closed loop Step-Down converter circuit raises difficulties with the stabilisation of both loops. Further, to achieve maximum efficiency in loops containing both Step-Up and Step-Down converters, the voltage drop between the Step-Up output and the Step-Down output should be minimised. However, given the battery voltage and output power of RFPA this can be difficult, often requiring the selection of a number of output voltages of the Step-Up converter leading to sub-optimal operating conditions.